1. Field of the Invention
This invention relates to a separable plastic fastener and, more particularly, to a separable plastic fastener which comprises two fastener pieces each including a base and a plurality of elastic engagement projections which project from the base, which fastener are arranged in a plurality of lateral and longitudinal rows and have heads of the same size and leg means, and to a method and an apparatus for manufacturing the separable plastic fastener.
2. Description of the Prior Art
A typical prior art separable fastener comprises two fastener pieces, one of which has a base and a plurality of J-shaped hooks provided densely on the base, and the other one of which has a base and a plurality of loops provided densely on the base. The two fastener pieces are fastened together by pressing them together so as to cause engagement of the hooks in the loops. However, this fastener requires two kinds of fastener pieces and an improved fastener has been proposed, which comprises two fastener pieces of the same kind having a base and a plurality of hooks and loops provided alternately and densely on the base (as disclosed in U.S. Pat. Nos. 2,717,437 and 3,130,111).
In the above prior art fasteners, thread-like loops are formed densely on the base. In addition, some of the loops are cut to form the hooks, thus requiring an additional step of manufacture.
In another well-known separable plastic fastener, elastic engagement projections are densely provided on a base in a plurality of lateral and longitudinal rows (as disclosed in U.S. Pat. Nos. 2,499,898, 3,192,589 and 3,408,705). In this kind of fastener, each engagement projection has a single leg projecting from the base and a head supported on top of the leg. The head is circular or polygonal in plan view and has a spherical, hemi-spherical, conical or umbrella-like profile. To fasten two fastener pieces together, they are pressed against each other with their surfaces provided with the engagement projections opposed. At this time, the head of each engagement projection of one of the fastener pieces is snap-engaged in a space between adjacent engagement projections of the other fastener piece. Thus, the bottom edges of the heads of one of the fastener pieces are hooked by those of the other fastener piece, and thus the two fastener pieces are fastened together. At this time, each head having its bottom center united to the top of a single leg can be tilted in any direction, and also the single leg itself can be flexed in any direction about its lower end united to the base. Therefore, when a force tending to unfasten the fastener pieces is applied to either of the fastened fastener pieces in any direction, the head of each engagement projection having snap-engaged in a space defined by adjacent engagement projections of the other fastener piece is tilted, and the single leg integral with the head is flexed. Thus, each engagement projection of either fastener piece can be readily detached from the space defined by adjacent engagement projections of the other fastener piece. In other words, this type of fastener provides low engagement maintaining force between the two fastener pieces. In addition, when either fastener piece is unfastened from the other fastener piece, the heads of each are liable to be broken off the top of the associated single leg.
Further, when each engagement projection of either fastener piece is thrust into the space between adjacent engagement projections of the other fastener piece, the head is shaken and tilted while the associated single leg is flexed. Therefore, attainment of the snap-engagement of the bottom edge portions of the engagement projection heads of the two fastener pieces pressed against each other is hard for the user to sense tactility.
Still further, with the fastener of this type the length of the leg (strictly speaking the height from the top surface of the base to the bottom of the head) is set to be greater than the height of the head from the bottom to the top thereof in order to make the leg highly flexible to thereby reduce the force of fastening the two fastener pieces resulting from the engagement of the projection head bottoms thereof or the force of unfastening the fastener pieces, i.e., the force applied thereto for pulling them apart. However, this will allow relative movement of the two bases to an extent proportional to the difference between the length of the leg of the engagement projection and the height of the head, thus permitting joggling.
Moreover, the single leg of each of the engagement projections integral with the flat base has its top united to the center of the head bottom. Therefore, the fastener cannot be injection molded with a two-piece mold having a male and a female die because the leg has its top shielded by the head and its bottom shielded by the base. This means that it is necessary to injection mold a plastic flat base having a plurality of integral eventual legs and form heads one by one by thermally fusing and deforming a top portion of each eventual leg. This requires much time and labor. In addition, the individual heads each formed by thermally deforming a top portion of each eventual leg are not uniform. It is thus difficult to obtain products uniform in the force of engagement in the fastened state.